<template>
  <ul class="list">
    <li
      class="item"
      v-for="item of letters"
      :key="item"
      :ref="item"
      @touchstart.prevent="handleTouchStart"
      @touchmove="handleTouchMove"
      @touchend="handleTouchEnd"
      @click="handleLetterClick"
    >{{item}}</li>
  </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  computed: {
    letters () {
      const letters = [];
      for (let i in this.cities) {
        letters.push(i);
      }
      return letters;
    }
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    };
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop;
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change', e.target.innerHTML);
    },
    handleTouchStart () {
      this.touchStatus = true;
    },
    handleTouchMove (e) {
      if (this.touchStatus) {
        // 节流
        if (this.timer) {
          clearTimeout(this.timer)
        }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20) - 1
          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },
    handleTouchEnd () {
      this.touchStatus = false;
    }
  }
};
</script>

<style lang="stylus" scoped>
@import '../../../assets/styles/varibles.stylus'

.list
  display flex
  flex-direction column
  justify-content center
  position absolute
  top 2rem
  right 0
  bottom 0
  width 0.4rem
  overflow hidden
  .item
    line-height 0.4rem
    text-align center
    color $bgColor
</style>
